1. Field
This invention relates to high temperature electrochemical cells employing liquid alkali metal anolytes and liquid chalcogen catholyte, and parcticularly to an electrochemical cell such as a secondary battery employing molten alkali metal, especially sodium, as an anolyte and a mixture of molten selenium as a catholyte.
2. Prior Art
Various types of electrochemical cells employing molten alkali metal electrode-reactants (anolytes) are known. These cells generally employ as an impervious electrolyte an ionically conductive ceramic, for example, beta"-alumina, or glass membranes through which alkali metal ions pass. Secondary batteries employing a molten sodium anolyte and a molten sulfur catholyte are described in U.S. Pat. Nos. 3,404,035 and 3,404,036 to Kummer, et al. A primary battery having a molten sodium reactant in contact with a sodium ion conductive ceramic membrane is described in U.S. Pat. No. 3,458,356 to Kummer, et al.
Selenium has been mentioned in various patents, for example, U.S. Pat. Nos. 3,476,602; 3,672,995 and 3,679,480 to Brown, et al. and U.S. Pat. No. 4,105,054 to Clever, et al., as a catholyte material. In Fischer, U.S. Pat. No. 4,127,705, selenium is described as an additive to the catholyte in minor quantities. The mention of selenium has generally been in reference with other catholytes in which sulfur is the preferred cathodic material and selenium, tellurium, tetracyanoethylene, para-thiocyanogen, ferricyanide, and the like are optional cathodic materials. As indicated in the Clever, et al. patent, a fused salt is interposed betwen the anodic material and the electrolyte. All experimentation referenced in the prior art patents known to applicant is related to sulfur as the cathodic material.
Substantially pure selenium has been employed as a catholyte by the instant inventors. Although certain advantages appear from the use of selenium as a catholyte, certain problems, such as high viscosity of the selenium-selenide mixture have become apparent. Also, some difficulties in wetting of solid components within the selenium has been suspected.
A great deal of work has been done on the sodium-sulfur battery. It has shown great potential as a power source for electric vehicles because of its power to weight ratio, which is about ten times better than conventional lead-acid batteries. The sodium-sulfur battery also has the potential of long life when operated within the proper parameters. These proper parameters may, however, be a hindrance to widespread commercialization of the sodium-sulfur battery.
The sodium-sulfur battery, based upon present performance, is projected to give a compact size vehicle a range of about 200 miles, moderate acceleration, and a recharge time of about eight hours. Such a vehicle would be a suitable urban vehicle. However, improvement of the range, acceleration and recharge rate would enhance considerably the usefulness of electric vehicles. The operating parameters of the sodium-sulfur battery remain an obstacle to such improvements.
The range of an electric vehicle is limited by the capacity of the battery powering it. The capacity is a function of degree of discharge. The sodium-sulfur battery can be discharged until the sulfur is converted to Na.sub. 2S.sub.3 (at 350.degree. C.). If it is discharged further, certain sulfides of sodium, normally An.sub.2 S.sub.2, are precipitated which become and remain insoluble even upon recharge. Repeated "deep-discharge" of a sodium-sulfur battery will ultimately render the battery useless.
The projected recharge rate for a sodium-sulfur battery; i.e., overnight recharging, is acceptable for an urban car with a range of 200 miles. However, it would be desirable for long distance travel to have a vehicle with a range of 300 to 400 miles, or more, with a recharging rate of one hours, which would permit recharging during meal breaks. Rapid recharging is limited by the critical current density which the solid electrolyte will tolerate. Electrolyte tubes of beta"-alumina, a particularly effective sodium ion conductor, have excellent life so long as recharging of a cell containing such electrolytes is done at a current density less than the critical current density. Critical current density increases for beta"-alumina electrolytes as temperature increases. Similarly, conductivity (the inverse of resistance) increases for such electrolytes with increasing temperature. Increasing the operating temperature of a sodium-sulfur cell above the usual temperature of 350.degree. C. is detrimental in other respects. Sulfur presents a troublesome corrosion problem at 350.degree. C.; at higher temperatures, the corrosion problem is greatly exacerbated. Sulfur-containing cells may become pressurized at more elevated temperatures. For a number of reasons, a pressurized cell is generally undesirable.
A further problem associated with sodium-sulfur batteries, particularly for some applications, has been the excessive weight required for a thermal management system. Since the temperature of a sodium-sulfur cell must generally be controlled at less than about 300.degree. C., and preferably below 350.degree. C., rapid discharge or recharging can create a great deal of heat energy which must be dissipated rapidly to maintain cell temperatures below 350.degree. C. The weight of the thermal management system for a conventional sodium-sulfur battery has been estimated to be at least as much as 25% of the total battery (cell plus heat exchangers, etc.).